Method for forming a GaN-based semiconductor light emitting device

ABSTRACT

A method for forming a GaN-based semiconductor layer includes the steps of: forming a ZnO buffer layer on one of a glass substrate and a silicon substrate; and epitaxially growing a GaN-based semiconductor layer on the ZnO buffer layer by using an electron cyclotron resonance--molecular beam epitaxy (ECR-MBE) method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting device,more particularly to a semiconductor light emitting device using aGaN-based material and a method for producing the semiconductor lightemitting device. The present invention also relates to a method forforming a GaN based semiconductor layer.

2. Description of the Related Art

GaN has a wide bandgap and light emitting diodes (LEDs) using GaN havebeen therefore known as semiconductor light emitting devices emittinglight in the region from blue to violet. Although semiconductor lightemitting devices emitting blue light are prospective key devices in anoptoelectronic field, there is a problem that it is very difficult togrow a GaN bulk crystal with high quality. Due to the problem, theresearch on the growth of GaN bulk crystals deals with the selection ofa suitable substrate and on a method for the GaN crystal deposition.

In this connection, a conventional method has tried to use asingle-crystal sapphire substrate for depositing the GaN layer thereonby applying the metal-organic chemical vapor deposition method(hereinafter referred to as the "MOCVD method"). In this method,however, a difficulty lies in depositing a GaN layer of high-qualitycrystallinity. This difficulty is attributed to a great differencebetween lattice constants of the sapphire substrate and GaN (aparticular difference between the lattice constants of the two is asgreat as about 16.1%), thereby developing crystal defects with adislocation density as large as 10⁸ to 10¹¹ /cm² in the deposited GaNlayer.

In recent years, a method has been proposed to cope with problems suchas that described above. In the proposed method, a polycrystalline oramorphous crystalline AlN layer is deposited as a buffer layer between asapphire substrate and a GaN layer in order to reduce the differencebetween the lattice constants of the single-sapphire substrate and theGaN layer, thereby enabling the deposition of a GaN layer ofhigh-quality crystallinity. It is also disclosed that using a ZnO layeras a buffer layer enables the deposition of the GaN layer on, inaddition to the single-crystal substrate, amorphous crystallinesubstrates such as a quartz glass substrate; and practical applicationsof this method are being developed (for example, Japanese UnexaminedPatent Publication No. 8-139361).

Even in these methods of the related art which form the GaN layer byusing a buffer layer, such as an AlN layer or a ZnO layer, on asubstrate, the MOCVD method is a mainstream method being used to depositthe GaN layer.

Despite technical development, however, conventional semiconductor lightemitting devices have the problems described below.

Specifically, the single-crystal sapphire substrate used hithertopredominantly as a substrate for a GaN layer increases the productioncosts because the substrate is higher-priced.

In addition, in both of the aforementioned methods of the related art,the MOCVD method is used. In the MOCVD method, however, a substrateneeds to be heated to a high temperature of 1,000 to 1,200° C. at thetime of vapor deposition to make use of a thermal decomposition reactionfor the crystal deposition. Such a high temperature causes the followingproblems. First, substrates that can be used for depositing a GaN layerthereon are limited to those having a high heat-resisting property.Second, because of the high temperature, a substrate receives a strongeffect resulting from a difference between the coefficients of thermalexpansion of the substrate and GaN. A particular example is a device inwhich the GaN layer is deposited on a single-crystal sapphire substrate.In this example, if the substrate on which a GaN layer is deposited atabout 1,000° C. is cooled from about 1,000° C. to an ambienttemperature, the substrate shrinks greater than the GaN layer due to adifference (about 34%) between coefficients of the single-crystalsapphire substrate and the GaN layer. This often causes distortions,cracks, and lattice defects in the GaN layer, which results indegradation of the crystal quality. As a result, it is difficult toobtain a device having a sufficient light emitting effect.

For the forgoing reasons, there is a need for a semiconductor lightemitting device that comprises a GaN-based layer having an excellentcrystallinity and a sufficient light emitting effect and that can beproduced at a low cost. There is also need for a method for producingthe semiconductor light emitting device and a method for forming aGaN-based semiconductor layer having an excellent crystallinity.

SUMMARY OF THE INVENTION

The present invention is directed to a device and a method thatsatisfies these needs.

The semiconductor light emitting device according to the inventioncomprises: a glass or silicon substrate having a softening point of 800°C. or less; a ZnO buffer layer provided on the glass substrate; andsemiconductor structure including at least one light emitting layer madeof a GaN-based semiconductor.

The light emitting layer is preferably formed by using an ECR-MBEmethod. The semiconductor light emitting device may further comprises anamorphous GaN-based semiconductor buffer layer between the ZnO bufferlayer and the light emitting layer. The light emitting layer may be madeof GaN semiconductor or InGaN semiconductor.

The method for forming a GaN-based semiconductor layer comprises thesteps of: forming a ZnO buffer layer on one of a glass substrate and asilicon substrate; and epitaxially growing a GaN-based semiconductorlayer on the ZnO buffer layer by using an electron cyclotronresonance--molecular beam epitaxy (ECR-MBE) method.

The method for producing a semiconductor light emitting device comprisesthe steps of: forming a ZnO buffer layer on one of a glass substrate anda silicon substrate; and epitaxially growing a light emitting layer madeof a GaN-based semiconductor layer on the ZnO buffer layer by using aECR-MBE method.

These methods may further comprise the step of forming an amorphousGaN-based semiconductor buffer layer on the ZnO buffer layer before theepitaxial growth step.

The epitaxial growth step is preferably performed at a temperature of850° C. or less and more preferably 700° C. or less. The ZnO layer maybe formed on the substrate, and the substrate is made of borosilicateand has a softening point of about 700 to 800° C.

The GaN-based semiconductor layer or the light emitting layer may be aGaN layer or a InGaN layer.

According to the present invention, since an ECR-MBE method is used toform a GaN-based layer, it is possible for nitrogen gas to be suppliedin a plasma-state by means of ECR and, by a level that is equivalent tothe excitation energy occurring therein, it is possible to lower thesubstrate temperature.

As a result, lowering the temperature at the time of layer depositionallows the use of materials having low melting points and, accordingly,increases the selection range for the substrate materials. For example,it is difficult to use lower-priced borosilicate glass materials for thesubstrate in the conventional methods, but it is now possible to usesuch materials, thereby reducing production costs for the semiconductorlight emitting device.

Also, lowering the substrate temperature prevents adverse effectsresulting from differences between the thermal expansion coefficients ofthe substrate and GaN. In addition, in comparison with other materials,GaN and a glass substrate are closer in regard to the thermal expansioncoefficient (specifically, the difference between the thermal expansioncoefficients of GaN and the glass substrate is about 10%; for reference,the difference between the thermal expansion coefficients of GaN and asapphire substrate is about 34%.) and have a higher property ofductility, cracks do not occur on the deposited GaN layer, therebyallowing a GaN layer of high quality and having high emission efficiencyto be produced.

For the purpose of illustrating the invention, there is shown in thedrawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangements and instrumentalities shown.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic view illustrating an ECR-MBE apparatus used forgrowing a GaN-based semiconductor layer and for producing asemiconductor light emitting device according to the present invention.

FIG. 2 is a cross-sectional view of a GaN layer grown by a methodaccording to a first embodiment of the present invention.

FIG. 3 is a time chart showing a deposition procedure for a GaN layeraccording to the first embodiment of the present invention.

FIG. 4 is a graph showing the photoluminescence spectrum obtained fromthe GaN layer according to the first embodiment of the presentinvention.

FIG. 5 is a cross-sectional view of an InGaN layer grown a by a methodaccording to a second embodiment of the present invention.

FIG. 6 is a graph showing the photoluminescence spectrum obtained fromthe InGaN layer according to the second embodiment of the presentinvention.

FIG. 7 is a schematic cross-sectional view illustrating a semiconductorlight emitting device according to a third embodiment of the presentinvention.

FIGS. 8A, 8B and 8C are schematic cross-sectional views illustratinglight emitting devices according to variations of the first, second andthird embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of the present invention areexplained in detail with reference to the drawings.

FIG. 1 is a schematic diagram of an ECR-MBE (Electron CyclotronResonance--Molecular Beam Epitaxy) apparatus to be used for growing aGaN-based semiconductor layer and producing a semiconductor lightemitting device according to the present invention. As this figureshows, the ECR-MBE apparatus comprises three chambers, i.e., a plasmageneration chamber 2, a deposition chamber 3, and a substrate exchangechamber 4.

In ECR-MBE apparatus, nitrogen gas is introduced into the plasmageneration chamber 2 at a predetermined flow rate by a mass-flowcontroller (not shown), and a microwave of 2.45 GHz and a magnetic fieldof 875 G are applied to the introduced nitrogen gas to cause electroncyclotron resonance (ECR), thereby generating plasma. The nitrogen-gasplasma that is generated is then forced by a divergent magnetic field toflow from the plasma generation chamber 2 into the deposition chamber 3.

The deposition chamber 3 comprises a substrate holder 12 with a heater,and a substrate 11 is held by the substrate holder 12. The depositionchamber 3 further comprises a voltage application means 13 for applyinga DC bias between a vacuum chamber and the substrate holder 12, whichconfigure the deposition chamber 3. This allows ionized source materialsto be effectively fixed on the substrate 11. Concurrently, the collisionof unnecessary ions with the substrate 11 is prevented to reduceion-emission damage, thereby allowing implementation of qualityimprovement for crystal layers formed on the substrate 11. Alsoinstalled therein are a Knudsen cell 14 for supplying a metal Ga as a Gasource and another Knudsen cell 15 for supplying an In metal as an Insource. These materials react with plasma-state nitrogen gas arranged toflow in from the plasma generation chamber 2 to form a GaN layer or anInGaN layer on the substrate 11. At this time, since the nitrogen gas issupplied in a plasma state, the temperature of the substrate 11 can belowered to a level equivalent to excitation energy. As a result, it ispossible to epitaxially grow a GaN-based layer at a temperature of 850°C. or less. As an evaluation device of the conditions of a layer on thesubstrate 11, a RHEED electron gun 16 and a screen device 17 areinstalled.

In the substrate exchange chamber 4, a substrate-carrying rod 18 forcarrying the substrate 11 into the deposition chamber 3 is installed.The substrate-carrying rod 18 is equipped with a heater for preheatingthe substrate. This rod 18 enables continuous formation of layers on thesubstrate and reduces the amount of evolving gas that evolves in thepreheating of the substrate 11 in the deposition chamber 3.

The deposition chamber 3 and the substrate exchange chamber 4 each havean exhaust system (for example, a turbo molecular pump and a oil rotarypump; not shown) and the two chambers are separated by means of a gatevalve 19. This allows the deposition chamber 3 to maintain a minimizedcontent of residual impurity molecules and the increased amount ofvacuum air.

First Embodiment

In this embodiment, as shown in FIG. 2, a GaN layer is formed on a glasssubstrate by using the ECR-MBE apparatus 1 described above. Thefollowing is a description of a substrate, materials, and a procedurewhich are used in this embodiment.

For a substrate in this embodiment, a low-priced borosilicate glasssubstrate (hereinafter, simply called glass substrate) 21 is used. Theborosilicate glass substrate preferably has a softening point of 800° C.or less and more preferably a softening point within the range of 700 to800° C. It is understood that the softening point of the glass substrateused in the present invention is at least higher than a maximumtemperature to which substrate holder 12 is heated. In this embodiment,a low-priced borosilicate glass having a softening point of about 775°C. is used.

Note that one of the features of the present invention is that the glasssubstrate having a relatively low softening point can be used as asubstrate on which a GaN-based semiconductor layer is epitaxially grown.Generally, the glass substrate having a high softening point isexpensive, and a quartz glass substrate having a softening point morethan 1000° C. is more expensive. There exists a great variety in termsof a softening point among substrates which are collectively referred asa glass substrate, and the fact that a glass substrate having a lowsoftening point can be employed for epitaxial growth of a GaN-basedsemiconductor layer brings a great advantage in realizing a commercialmass production of a semiconductor light emitting device emitting bluelight.

On this glass substrate 21, using a method, such as an RF magnetronsputtering method, a ZnO buffer layer 22 at a thickness of about 3 μm isdeposited. This layer 22 is a polycrystalline layer oriented to the caxis.

The materials used to deposit the GaN layer are a metal Ga of a purity8N (99.999999%) as a Group III material and nitrogen of a purity 5N as aGroup V gas. First, a glass substrate 21 is set onto thesubstrate-carrying rod 18 of the ECR-MBE device 1. Then, prebaking isperformed to eliminate free water and adsorption gas from the glasssubstrate 21 or the ZnO buffer layer 22. The glass substrate 21 istransferred into the deposition chamber 3, and thermal cleaning at 700°C. is therein performed for the substrate 21 for 30 minutes so that acleaned surface of a ZnO buffer layer 22 is produced. Then,low-temperature deposition is performed under the conditions ofdeposition shown in Table 1, for 20 minutes, to grow a GaN buffer layer23 of about 20 nm in thickness. This buffer layer 23 deposited at a lowtemperature is amorphous and is intended to improve the crystallinity ofa single crystal GaN layer 24 which will be deposited in a later stepand may be omitted. The deposition chamber 3 is maintained at about 10⁻⁷Torr.

                  TABLE 1                                                         ______________________________________                                        Substrate                                                                              Ga Cell   Nitrogen  Microwave                                                                             Substrate                                  Temperature Temperature Flow Rate Power Bias                                ______________________________________                                        450° C.                                                                         850° C.                                                                          30 sccm   80 W    +18 V                                    ______________________________________                                    

After the buffer layer 23 is deposited at a lower temperature, a GaNlayer 24 is epitaxially grown for 120 minutes at a pressure of about10⁻⁷ Torr under the conditions shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Substrate                                                                              Ga Cell   Nitrogen  Microwave                                                                             Substrate                                  Temperature Temperature Flow Rate Power Bias                                ______________________________________                                        680° C.                                                                         860° C.                                                                          39 sccm   120 W   +20 V                                    ______________________________________                                    

As a result of carrying out the aforementioned procedure, the layeredstructure shown in FIG. 2 is produced. A time chart of this depositionprocedure for the GaN layer is shown in FIG. 3. As is apparent in FIG.3, in the series of deposition steps of this embodiment, the substratetemperature can be maintained at 700° C. or lower (although notdescribed above, when the RF magnetron sputtering method is applied, thedeposition can be performed at about 200° C.). This allows materials oflower melting point and softening point to be used for the substrate,increasing the selection range for the substrate material. It is verydifficult to employ a glass substrate having such a low softening pointby a conventional method.

Optical properties of the GaN layer grown by the aforementioned methodwill be discussed. As an optical-property evaluation method, aphotoluminescence spectrum with an excitation light source of He-Cdlaser is measured at a temperature of 77K or lower. The measurementresult is shown in FIG. 4. In FIG. 4, the horizontal axis represents thelight emission wavelength λ, and the vertical axis represents theluminous intensity (unit: a.u.). As can be seen from this figure, forthe GaN layer produced in this embodiment, an emission spectrum can beconfirmed mainly in the vicinity of a band end (360 nm).

According to the inventors' further study, it has been found that ZnO islikely vaporized at a temperature of about 900° C. or more even under alow vacuum condition. It is, therefore, thought that a ZnO buffer layermight disappear during the formation of a GaN layer by a conventionalMBE method or CVD method which must employ a temperature of 900° C. ormore and it is uncertain that the ZnO buffer layer exists so as to trulyact as a buffer layer.

On the contrary, according to the present invention, the substratetemperature during the formation of a GaN layer can be successfullyreduced to about 700° C. or less by using a ECR-MBE method. Thiseliminates the decomposition or vaporization of the ZnO buffer layer.Accordingly, it is possible to form a GaN layer having a high-qualitycrystallinity due to the existence of the true ZnO buffer layer.

Second Embodiment

In this embodiment, as shown in FIG. 5, an InGaN layer is epitaxiallygrown on a glass substrate by using the ECR-MBE apparatus 1 describedabove. The following describes a substrate, materials, and a procedurewhich are used in this embodiment.

In this embodiment, the materials used for depositing an InGaN layer area metal Ga of a purity 8N (99.999999%) as a Group III material andanother metal In of the same purity as the In metal. Other materials andthe substrate used in this embodiment are the same as for the firstembodiment.

Regarding a deposition procedure, the deposition conditions for an InGaNbuffer layer 33 depositing being deposited at a low temperature andthose for an InGaN layer 34 are respectively set to those shown in Table3. Other conditions are the same as in the first embodiment.

                  TABLE 3                                                         ______________________________________                                                       Ga      In    Nitrogen                                                                             Micro-                                       Substrate Cell cell Flow wave Substrate                                       Temp. Temp. Temp. Rate Power Bias                                          ______________________________________                                        Buffer                                                                              450° C.                                                                         850° C.                                                                        640° C.                                                                      30 sccm                                                                               80 W +18 V                                 Layer                                                                         InGaN 680° C. 860° C. 640° C. 30 sccm 120 W +20 V                                                 Layer                              ______________________________________                                    

As a result of carrying out the aforementioned procedure, the InGaNlayer 5 is epitaxially grown on the glass substrate 31. A time chart ofthis deposition procedure for the GaN layer is shown in FIG. 3. In theseries of deposition steps of this embodiment as well, the substratetemperature can be maintained at 700° C. or lower.

The optical properties of the InGaN layer produced in the above stepsare shown in FIG. 6. As an optical-property evaluation method, the samemethod as in the first embodiment is used. As can be seen from thisfigure, for the InGaN layer produced in this embodiment, an emissionspectrum can be confirmed mainly in the vicinity of a band end (380 nm).

Third Embodiment

FIG. 7 shows a schematic cross-sectional view of a semiconductor lightemitting device 40. The semiconductor light emitting device comprises aglass substrate 41, a ZnO buffer layer 42 provided on the glasssubstrate 41 and a semiconductor structure 50 provided on the ZnO bufferlayer 42. The semiconductor structure 50 includes an n-GaN claddinglayer 44, a p-GaN cladding layer 46 and an InGaN active layer 45 stackedbetween the n-GaN cladding layer 44 and the p-GaN cladding layer 46. Thesemiconductor structure 50 further includes a GaN buffer layer 43 so asto be interposed between the n-GaN cladding layer 44 and the ZnO bufferlayer 42. An n-type electrode 47 is formed on a side surfaces of then-GaN cladding layer 44, the n-GaN cladding layer 44 and the and the ZnObuffer layer 42. A p-type electrode 48 is formed on the top surface ofthe p-GaN cladding layer 46.

The semiconductor structure 50 is formed on ZnO buffer layer 42 in thesame way as explained in the first and second embodiments using Znmetal, Mg metal and Si as sources for doping materials. Morespecifically, the non-doped amorphous GaN buffer layer 43 having athickness of 0.02 μm is first formed on the ZnO buffer layer 42. Thenthe Si-doped n-GaN cladding layer 44 having a thickness of 3 μm and animpurity concentration of 1×10¹⁸ cm⁻³, the Zn-doped InGaN active layer45 having a thickness of 0.01 μm and an impurity concentration of 1×10²⁰cm⁻³ and the Mg-doped p-GaN cladding layer 46 having a thickness of 0.8μm and an impurity concentration of 1×10¹⁷ cm⁻³ are successively formedon the GaN buffer layer 42.

According to this embodiment, the semiconductor structure 50 having anexcellent crystallinity can be formed at a temperature of 700° C. orless on the glass substrate 41. Therefore, the semiconductor lightemitting device 40 can emit blue light without suffering from theproblems associated with the conventional art.

Other Embodiments

It is a matter of course that the present invention is not limited tothe deposition conditions described in the aforementioned embodimentsand it is variable within an intended range described herein. Forexample, in the embodiments described above, a borosilicate glasssubstrate is used, but, so as not to be limited to this, other siliconsubstrates, which are also low-priced substrates, can be used, as shownin FIGS. 8A, 8B and 8C, which corresponds respectively to FIGS. 2, 5 and7, except for the use of silicon substrates 21a, 31a and 41a rather thanthe glass substrates 21, 31 and 41. In this case, other IC parts can beformed on the same substrate on which a GaN layer is deposited.

In the aforementioned embodiments, the GaN-based layer is formed atabout 680° C. The GaN-based layer, however, may be formed at atemperature in the range of about 400 to 500° C. so as to furtherimprove the crystallinity. In the case, the glass substrate having asoftening point of less than 700° C. can be employed. On the other hand,in the case where a silicon substrate is used, a GaN-based layer may beformed at a temperature of 850° C. or less as a silicon substrate doesnot melt or deform at the temperature.

In addition, although formations of the GaN layer and the InGaN layerand a semiconductor light emitting device having the GaN layer and theInGaN layer are explained as embodiments of the present invention, it iswell understood that the present invention can be applied to a formationof layer of Ga_(1-x) In_(x) N, Ga_(1-x) Al_(x) N, Ga_(1-x) B_(x) N andmixed crystal thereof, i.e., GaN-based materials.

Further, although the semiconductor light emitting device according tothe third embodiment has a double heterostructure, the semiconductorlight emitting device may be has a single heterostructure or ahomostructure and has a light emitting diode structure or a laserstructure as long as the semiconductor light emitting device includes aGaN-based layer which emits a light in the region from blue to violet,as a light emitting layer.

While preferred embodiments of the invention have been disclosed,various modes of carrying out the principles disclosed herein arecontemplated as being within the scope of the following claims.Therefore, it is understood that the scope of the invention is not to belimited except as otherwise set forth in the claims.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A method for forming a GaN-based semiconductorlayer, comprising the steps of:forming a ZnO buffer layer on a substrateconsisting of either glass or silicon; and epitaxially growing aGaN-based semiconductor layer on the ZnO buffer layer by using anelectron cyclotron resonance--molecular beam epitaxy (ECR-MBE) method.2. The method for forming a GaN-based semiconductor layer according toclaim 1, further comprising the step of forming an amorphous GaN-basedsemiconductor buffer layer on the ZnO buffer layer before the epitaxialgrowth step.
 3. The method for forming a GaN-based semiconductor layeraccording to claim 1, wherein the epitaxial growth step is performed ata temperature of 850° C. or less.
 4. The method for forming a GaN-basedsemiconductor layer according to claim 3, wherein the ZnO layer isformed on the glass substrate, and the glass substrate is made ofborosilicate and has a softening point of about 700 to 800° C.
 5. Themethod for forming a GaN-based semiconductor layer according to claim 1,wherein the GaN-based semiconductor layer is a GaN layer.
 6. The methodfor forming a GaN-based semiconductor layer according to claim 1,wherein the GaN-based semiconductor layer is a InGaN layer.
 7. A methodfor producing a semiconductor light emitting device, comprising thesteps of:forming a ZnO buffer layer on a substrate consisting of eitherglass or silicon; and epitaxially growing a light emitting layer made ofa GaN-based semiconductor layer on the ZnO buffer layer by using anelectron cyclotron resonance--molecular beam epitaxy (ECR-MBE) method.8. The method for producing a semiconductor light emitting deviceaccording to claim 7, further comprising the step of forming anamorphous GaN-based semiconductor buffer layer on the ZnO buffer layerbefore the epitaxial growth step.
 9. The method for producing asemiconductor light emitting device according to claim 7, wherein theepitaxial growth step is performed at a temperature of 850° C. or less.10. The method for producing a semiconductor light emitting deviceaccording to claim 9, wherein the ZnO layer is formed on the glasssubstrate, and the glass substrate is made of borosilicate and has asoftening point of about 700 to 800° C.
 11. The method for producing asemiconductor light emitting device according to claim 7, wherein thelight emitting layer is a GaN layer.
 12. The method for forming aGaN-based semiconductor layer according to claim 7, wherein the lightemitting layer is a InGaN layer.